Best material for heat exchanger on a space ship

[Melbourne Guy]

Looking for the best material for a heat exchanger exposed to vacuum in a space ship. I've initially gone with radiator vanes that are a ceramic blend and typically operate at twenty-five hundred degrees, but then I started wondering if there is a better material...and now I'm here, hoping for interesting answers!

 

[Rive]

operate at twenty-five hundred degrees

Which kind of degree?
Pretty high anyway. Where is the heat coming from? Regarding efficiency it's ... just weird.
Ceramics are OK I guess, but what will deliver the heat? Liquid metal? Some gas? How is this coming together?

 

[Melbourne Guy]

Hey @Rive, it's an incidental point in the plot, so I haven't elaborated the entire system, and I'm using SI units in the novel but this is a conversation on the ship's bridge so the players do not declare which units are in use.

Regarding efficiency it's ... just weird.

How so?

 

[Astronuc]

twenty-five hundred degrees

°F, °R, °C or K? The radiator rejects heat to space, so that implies a heat source of a greater temperature. 2500°F would be manageable with a Mo alloy (mp 2883 K), while Nb (mp 2741 K) might be too soft. If 2500°C (Nb would liquid, Mo would be creeping or flowing), the one would have to use a Ta or W alloy (Ta, mp 3269 K; W, mp 3683 K). Ref: W. D. Klopp, "Technology Status of Molybdenum and Tungsten Alloys," Proceedings of the first symposium on Space Nuclear Power Systems, M.S. El-Genk and M. D. Hoover, Eds., Orbit Book Company, 1985. The symposium was held in Albuquerque, New Mexico.

In terms of mass, a Ta-alloy would be preferable to a W-alloy. One might consider one of the ASTAR alloys: 811C, 1211C or 1511C, where the 8, 12 and 15 represent the weight percent of tungsten in the alloy, each with 1% Re, 0.7% Hf (~1%) and 0.025% by weight (mass). The Re provides for some ductility and solid solution strengthening, and HfC provides creep strength (dispersion strengthening) at high temperature.

 

[Melbourne Guy]

°F, °R, °C or K?

I'm not aware of the 'R' units, @Astronuc, but thank you for the detailed suggestions, and note that the other side of the exchanger is even hotter. I'll take a look at the ASTAR alloys, I've not come across them before

 

[Astronuc]

I'm not aware of the 'R' units

°R is the absolute scale, Rankine, related to °F, as K is related to °C. T(°R) = T(°F) + 459.67°, and 1 K = 1.8°R, or 1.8*T(K) = T(°R). °F, °R are in the English system (lbm, ft, s), while °C, K are used in SI or MKS system. The 2500 reminded me of 1371°C (2500°F) or about the melting point of many steels. Ni-alloys are melting by around 1430°C, so at 1371°C, they would have very little strength and would probably be 'flowing' under low to moderate stress.

From Melbourne, I suspect one is thinking in °C or K, but there is a difference. 2500 K < 2500°C in terms of absolute temperature. When applying the Stefan-Boltzmann law, one would normally use absolute temperature. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html

It might be difficult to locate literature on the ASTAR alloys. Two names to look for are Eugene. E. Hoffman (ORNL and AEC) and R. W. (Bill) Buckman (Westinghouse Astronuclear Laboratory, a division of Westinghouse Electric Corporation) back in the 1960s and 1970s.

https://ntrs.nasa.gov/citations/19710010916
https://ntrs.nasa.gov/citations/19750005014
https://ntrs.nasa.gov/citations/19740008105

I'll take a look at the ASTAR alloys, I've not come across them before

They are somewhat esoteric as far as alloys go.

 

[Melbourne Guy]

They are somewhat esoteric as far as alloys go.

Esoteric is good, this is sci-fi, after all

 

[Rive]

it's an incidental point in the plot

Then maybe it's better to leave it like that.

How so?

It's just absurdly high. Usually a (thermal) power plant has a 'hot leg' around a few hundred Celsius at most, and your spacecraft has its 'cold leg' higher without utilization?
Or, if it is the 'cold leg' what's that high, then what would be the 'hot leg' there?
Or is it some heat pump? But that also has some efficiency, the higher the temperature, the worse...
High temperature is good to lose many heat on a small surface, so it can be reasoned, but it's also lost energy and: trouble.

 

[Melbourne Guy]

It's just absurdly high.

It's dumping the heat of the bubble drive, which folds spacetime like an Alcubierre warp drive and that generates significant thermal load! Or at least, I've now decided it does, based on this thread